Identification and Secretion of Effectors From the <i>Pseudomonas syringae</i> Type III Secretion System

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Abstract

The hrp pathogenicity island (PAI) of Pseudomonas syringae encodes a type III secretion system (TTSS) that translocates virulence proteins, called effectors, into plant cells. The whole array of effectors of different P. syringae stains and their activities inside the host are not known. Furthermore, the manner in which effectors are selected for secretion, and how the process is regulated are not clear in P. syringae. This study identified a novel effector from P. syringae pv. syringae Psy61 using a genomic screen. The effector was a 375 aa protein of 40.5 kDa that was designated HopPsyL. A hopPsyL::kan mutant of Psy61 exhibited strongly reduced virulence in Phaseolus vulgaris cv. Kentucky Wonder, but did not appear to act as a defense response suppressor. The ectopically expressed gene reduced the virulence of P. syringae DC3000 transformants in Arabidopsis thaliana Col-0. HopPsyL appears to be a novel TTSS-dependent effector that functions as a host-species-specific virulence factor in Psy61. In addition, this study reports that TTSS-dependent effectors are subject to the proteolytic degradation by Lon that appears to be rate-limiting to secretion. TTSS-dependent secretion of these effectors could be detected from the Lon mutants. This study found that a primary role for chaperones in P. syringae appeared to be protection of effectors from Lon-mediated degradation prior to secretion. Distinct Lon-targeting and chaperone-binding domains were identified in at least one effector. The results imply that Lon is involved at two distinct levels in the regulation of the P. syringae TTSS: regulation of assembly of the secretion apparatus and modulation of effector secretion. Interestingly, degradation of P. syringae effectors was also retarded by the presence or expression of the P. syringae TTSS. The protection from Lon-mediated degradation was not due to the assembly of the TTSS. Rather, the results suggest the existence of a stabilizing factor harbored within the hrp cluster. This study proposed that this factor functions as a general chaperone for type III secretion in P. syringae.